A fuel cell generally includes a stack of elementary cells, within which an electrochemical reaction takes place between two reagents that are introduced continuously.
The fuel, for example hydrogen, is brought into contact with the anode while the oxidant, generally oxygen, is brought into contact with the cathode.
The anode and the cathode are separated by an electrolyte, of the proton exchange membrane type.
In the case of a hydrogen/oxygen stack, the anode is the oxidation location of the hydrogen, and the oxygen reduction is done at the cathode. An electrochemical reaction then takes place that creates electrical energy.
A catalyst, for example platinum grains, is generally present at the anode to improve the output of the electrochemical half-reaction.
However, the catalytic sites are particularly sensitive to CO, which tends to build up there. Thus, it is known that a CO concentration of several ppm is sufficient to poison the catalytic sites.
This poisoning results in particular in a substantial increase in the anode potential, which causes a drop in the voltage at the terminals of the cell. The performance of the cell is then greatly diminished.
However, the hydrogen used is commonly obtained by reforming a hydrocarbon compound (oil, natural gas, coal, biofuel . . . ). Using this method, a hydrogen-rich reformate gas, but also containing a CO concentration that can go from several tens of ppm to several percent.
Also, it is important to be able to quickly determine the CO concentration present in a gas intended to supply the anode of a fuel cell, using a device that is very simple to implement.
U.S. Pat. No. 6,488,836 describes an electrochemical device comprising a detection electrode separated from a counter electrode by a solid electrolytic membrane, each electrode being connected to a control unit.
When the detection electrode is in contact with gas containing hydrogen and CO, a voltage source applies a potential difference between the two electrodes so as to cause the oxidation of the hydrogen at the detection electrode, the transfer of the protons through the electrolyte, and the reduction of the protons at the counter electrode.
The voltage source also varies the difference in potential via a rectangular function. More precisely, the potential applied to the detection electrode alternates between a low potential allowing the adsorption of the CO by the detection electrode and a high potential causing oxidation of the CO.
The control unit measures, via an ammeter, the current drop caused by the applied voltage variation, calculates the rate of decrease of the corresponding current, then determines the CO concentration using a calibration curve indicating the CO concentration as a function of the rate of decrease.
This device does, however, have the drawback of having to determine a certain number of parameters beforehand intended to control the adsorption and desorption of the CO by the detection electrode in a controlled manner, using an electrical means. It is in fact necessary to define the frequency and the minimum and maximum threshold values of the applied voltage variation.
Moreover, these parameters can depend on operating conditions, physical characteristics of the detection electrode, the surface area of the active zone, and the CO concentration in the gas. The operation of the device is then not optimized.